The present invention relates to a method for conveying sheets in which a sheet guided upon sliding downward under its own weight is positioned with its front edge orthogonal to the direction of advancement of the sheet and is then conveyed with two opposed rollers or a pair of rollers. Such a method is hereinafter referred to as "a sheet orthogonal-conveying method". The invention also relates to a device for practicing the method, hereinafter referred to as "a sheet orthogonal-conveying device".
An operation of conveying a sheet with a pair of rollers, the sheet sliding under its own weight, is used in releasing exposed X-ray picture film sheets from a cassette 1 as shown in FIG. 1. A released film sheet 2 slides down a guide member 3 and its front edge is introduced between a pair of rollers 4 and 5. The film sheet 2 is then conveyed by rotation of the rollers 4 and 5 to the following process position such as for exposure printing with an ID card or developing. In this connection, it is desirable that the front edge of the film sheet 2 conveyed with the pair of rollers be orthogonal to the direction of advancement or conveyance of the sheet. This is essential for the case where the sheet is subjected to exposure printing with an ID card so that the image is printed at a predetermined position on the sheet.
A conventional method for conveying a sheet which has slid under its own weight with a pair of rollers with the front edge of the sheet orthogonal to the direction of advancement of the sheet is such that the front edge of the sheet is made orthogonal with the direction of advancement of the sheet when it is stopped along the nip region of the two rollers. The sheet is conveyed by rotation of the rollers which is started following a time delay after the front edge has been brought into contact with the rollers. This method is an application of the technique described in the specification of Japanese Laid-Open patent application No. 112366/1974, for instance.
However, the conventional method is disadvantageous in the following points. In the case where a sheet slides down at high speed or a sheet is thin and rigid to some extent, sometimes the sheet penetrates unevenly into the nip region of the rollers. That is, one of the right and left ends of the front edge of the sheet goes into the nip region more deeply than the other with the result that the front edge is not orthogonal with the direction of advancement of the sheet.
In order to overcome this difficulty, a technique has been disclosed in Japanese Laid-Open patent application No. 79682/1977. With this technique, after the front edge of a sheet strikes against a pair of rollers, the rollers are turned in the opposite direction to prevent the front edge from going into the nip region of the rollers deeply and to set the front edge straightly along the nip region, that is, to make the front edge orthogonal to the direction of advancement of the sheet. Then the rollers are turned in the forward direction to convey the sheet. In this method, in the course of making the front edge of the sheet orthogonal to the direction of advancement of the sheet, sometimes the corners of the sheet are shifted along the nip region of the rollers in the wedge-shaped space of the nip region. Because of this, it takes a relatively long time to make the front edge orthogonal to the direction of advancement of the sheet because of frictional resistance which is caused when the corners of the sheet are shifted as described above. With a sheet curled to some extent, sometimes one of the ends of the front edge of the sheet goes further into the wedge-shaped space than the other and this state is maintained unchanged for a time with the result that the front edge of the sheet is immediately not made orthogonal to the direction of advancement of the sheet.
Accordingly, an object of the invention is to provide a sheet orthogonal-conveying method in which the difficulty accompanying a conventional method that the front edge of a sheet sliding down under its own weight goes into the nip region of a pair of rollers deeply is eliminated and even a sheet curled to some extent can be conveyed with its front edge orthogonal to the direction of advancement of the sheet.